1. Field of the Invention
The present invention relates to a positive electrode active material for nonaqueous electrolyte secondary batteries and a production method thereof, and a nonaqueous electrolyte secondary battery using the positive electrode active material.
2. Description of the Related Art
In recent years, with the wide adoption of portable electronic devices such as mobile phones and laptop computers, the development of small and lightweight secondary batteries having high energy density is strongly desired. Further, the development of high power secondary batteries as batteries for electric cars including hybrid cars is strongly desired.
Examples of secondary batteries satisfying such demands include nonaqueous electrolyte secondary batteries typified by lithium ion secondary batteries. Such lithium ion secondary batteries are composed of a negative electrode, a positive electrode, an electrolyte, etc., and materials capable of intercalation and deintercalation of lithium ions are used for the active materials of the negative electrode and the positive electrode.
The nonaqueous electrolyte lithium ion secondary batteries are now being actively studied and developed. Above all, lithium ion secondary batteries using a layered or spinel lithium-nickel composite oxide as a positive electrode material allow a high voltage of 4-V class to be obtained, and therefore are being put into practical use as batteries having high energy density.
Main examples of materials proposed so far include lithium cobalt composite oxide (LiCoO2) that is comparatively easily synthesized, lithium nickel composite oxide (LiNiO2) using nickel that is less expensive than cobalt, lithium nickel cobalt manganese composite oxide (LiNi1/3Co1/3Mn1/3O2), and lithium manganese composite oxide (LiMn2O4) using manganese.
Among these, lithium-nickel composite oxide is gaining attention as a material having good cycle characteristics and low resistance and allowing high power to be obtained, where the resistance reduction that is necessary for power enhancement has been regarded as being important in recent years.
As a method for achieving the aforementioned resistance reduction, addition of different elements is used, and transition metals capable of having high valence such as W, Mo, Nb, Ta, and Re are considered to be useful, in particular.
For example, Japanese Patent Laid-Open No. 2009-289726 proposes a lithium transition metal compound powder for lithium secondary battery positive electrode materials containing one or more elements selected from Mo, W, Nb, Ta, and Re in an amount of 0.1 to 5 mol % with respect to the total molar amount of Mn, Ni, and Co, where the total atomic ratio of Mo, W, Nb, Ta, and Re with respect to the total of Li and the metal elements other than Mo, W, Nb, Ta, and Re on the surface portions of primary particles is preferably 5 times or more the atomic ratio of the whole primary particles.
According to this proposal, it is considered that the cost reduction, high safety, high load characteristics, and improvement in powder handleability of the lithium transition metal compound powder for lithium secondary battery positive electrode materials can be achieved all together.
However, the aforementioned lithium transition metal compound powder is obtained by pulverizing a raw material in a liquid medium, spray drying a slurry in which the pulverized materials are uniformly dispersed, and firing the obtained spray-dried material. Therefore, some of different elements such as Mo, W, Nb, Ta, and Re are substituted with Ni disposed in layers, resulting in a reduction in battery characteristics such as battery capacity and cycle characteristics, which has been a problem.
Further, Japanese Patent Laid-Open No. 2005-251716 proposes a positive electrode active material for nonaqueous electrolyte secondary batteries having at least a lithium transition metal composite oxide with a layered structure, wherein the lithium transition metal composite oxide is present in the form of particles composed of either or both of primary particles and secondary particles as aggregates of the primary particles, and wherein the particles have a compound including at least one selected from the group consisting of molybdenum, vanadium, tungsten, boron, and fluorine at least on the surface.
With that, it is claimed that the positive electrode active material for nonaqueous electrolyte secondary batteries having excellent battery characteristics even in more severe use environment is obtained, and that the initial characteristics are improved without impairing the improvement in thermostability, load characteristics, and output characteristics particularly by having the compound including at least one selected from the group consisting of molybdenum, vanadium, tungsten, boron, and fluorine on the surface of the particles.
However, the effect by adding the at least one element selected from the group consisting of molybdenum, vanadium, tungsten, boron, and fluorine is to improve the initial characteristics, that is, the initial discharge capacity and the initial efficiency, where the output characteristics are not mentioned. Further, according to the disclosed production method, the firing is performed while the additive element is mixed with a heat-treated hydroxide together with a lithium compound, and therefore the additive element is partially substituted with nickel disposed in layers to cause a reduction in battery characteristics, which has been a problem.
Further, Japanese Patent Laid-Open No. H11-16566 proposes a positive electrode active material in which the circumference of the positive electrode active material is coated with a metal containing at least one selected from Ti, Al, Sn, Bi, Cu, Si, Ga, W, Zr, B, and Mo and/or an intermetallic compound obtained by combining a plurality of these elements, and/or an oxide.
It is claimed that such coating can ensure the safety by absorbing oxygen gas, but there is no disclosure on the output characteristics. Further, the disclosed production method involves coating using a planetary ball mill, and such a coating method causes physical damage on the positive electrode active material, resulting in a reduction in battery characteristics.
Further, Japanese Patent Laid-Open No. 2010-40383 proposes a positive electrode active material heat-treated while a tungstate compound is deposited on composite oxide particles mainly composed of lithium nickelate and having a carbonate ion content of 0.15 weight % or less.
According to this proposal, since the tungstate compound or a decomposition product of the tungstate compound is present on the surface of the positive electrode active material, and the oxidation activity on the surface of the composite oxide particles during charge is suppressed, gas generation due to the decomposition of the nonaqueous electrolyte or the like can be suppressed, but there is no disclosure on the output characteristics.
Further, the disclosed production method is to deposit a solution in which a sulfuric acid compound, a nitric acid compound, a boric acid compound, or a phosphate compound serving as a deposition component is dissolved in a solvent together with the tungstate compound, on the composite oxide particles that are preferably heated to at least the boiling point of the solution in which the deposition component is dissolved, where the solvent is removed within a short time, and therefore the tungsten compound is not sufficiently dispersed on the surface of the composite oxide particles and is not uniformly deposited, which has been a problem.
Further, improvements in power enhancement by lithium nickel composite oxide have also been made.
For example, Japanese Patent Laid-Open No. 2013-125732 proposes a positive electrode active material for nonaqueous electrolyte secondary batteries having fine particles containing lithium tungstate represented by any one of Li2WO4, Li4WO5, and Li6W2O9 on the surface of a lithium-nickel composite oxide composed of primary particles and secondary particles formed by aggregation of the primary particles, where high power is supposed to be obtained together with high capacity.
Although the power is enhanced while the high capacity is maintained, further enhancement in capacity is required.
In view of such problems, it is an object of the present invention to provide a positive electrode active material for nonaqueous electrolyte secondary batteries which allows high power together with high capacity to be obtained when used as a positive electrode material.